A. Field of the Invention
This invention relates to remote access to the public switched telephone network ("PSTN") by way of a wireless cellular telephone radio link. More specifically, the invention uses a cellular system to make a virtual connection between the dedicated lines of multiple subscribers of the PSTN and each subscriber's telephone instrument at the subscriber's home or office. The system provides each subscriber with direct access to the public switch, including all call progression cues, such as a dial tone, provided by the PSTN. Conversely, the system allows the DTMF digits entered by the user to be transmitted directly back to the PSTN. The invention thus operates with a conventional telephone in a manner consistent with normal telephone service, with the wireless portion of the transmission being transparent to the subscriber.
B. Description of the Prior Art
There are many instances in which the provision of an ordinary connection between a subscriber's home or office telephone and the PSTN, based upon a physical connection such as traditional copper wire or optical fiber, is hampered by the cost of installing the physical connection. This occurs in developing countries in which the cost of installing the whole infrastructure is prohibitive or in rural areas where it is not cost effective to make the investment in physical connections in an area of low population. In some instances, the terrain provides obstacles or hindrances to effective installation.
It may be, however, that implementation of a wireless cellular system is cost effective in such circumstances. However, it is often undesirable from a subscriber's viewpoint to go through the additional procedures required when placing a cellular call, such as initiating a send signal after dialing the destination number and experiencing some delays in signal processing which are not encountered in typical PSTN call procedures.
It has been suggested to provide a radio link between a telephone user and the public switch using a remote station transceiver controlled by a push-to-talk key actuated by the user. U.S. Pat. No. 3,912,875 (Katz) describes a system in which a wireless base station is physically connected to the public switching system. The base station provides a radio communication link with a remote station. When originating a call from the remote station, the user lifts the telephone hand set off the cradle. Associated circuitry automatically places the remote station transceiver in a transmit mode. An initiate signal is transmitted to the base station. A base station control circuit, upon reception of a pre-selected number of initiate signal pulses, connects the base station with the telephone switching system. The remote operator then dials the telephone number and the base station generates a dial pulse train and transmits it to the telephone switching system. The PSTN then makes the appropriate connections to the destination telephone number.
The Katz system, which predated the development of reliable cellular telephone systems, provides a radio link between the public switch and a remote station, however, the Katz base station and the remote station must be specially designed for this purpose. More specifically, the Katz stations, for example, communicate in only one direction at a given time. Each station must switch between the transmit and receive mode. The switching is controlled by "push-to-talk" circuits in the stations. Further, the Katz remote station uses a dial signal generator for transmitting dial signal pulses corresponding to the telephone number dialed by the user. Thus, a specially designed base station is required to convert these pulses to dial signals to which the PSTN will respond. The system allows a single user at a time to access a telephone line which is shared by remote stations.
Subsequently, when cellular telephone systems were developed, it was suggested to use a cellular telephone transceiver to place a call which appears to the user to be a PSTN call, but in which the cellular system simulates the PSTN signals and makes the physical connection to the PSTN. U.S. Pat. No. 4,658,096 (West, Jr. et al.) describes a device which simulates the PSTN such that the subscriber can use the telephone in a conventional manner, although a cellular call is being placed.
More specifically, as described in that patent, the subscriber's conventional telephone set is connected to a unit which responds to an off-hook condition by separately generating its own dial tone for the user. The user responds by dialing a telephone number in the conventional fashion. The West, Jr. et al. device then converts these tone-dial or pulse-dial numbers into a digital data bit stream and passes this data on to a cellular transceiver. The system also counts the digits entered to determine when the last digit has been entered depending, for example, upon whether a local or long distance call is being placed. The unit then provides a "SEND" signal to the transceiver. In this way, the user is not required to press a SEND button.
The call proceeds essentially as a normal cellular call between a subscriber and the dialed line. The cellular system thus completes the connection in accordance with conventional cellular technology by assigning voice channels for the communication. The West, Jr. et al. unit also responds to incoming calls received at the transceiver and rings the subscriber's telephone.
The West, Jr. et al. patent involves a complex circuit which duplicates and simulates PSTN signals. There remains a need, therefore, for a relatively simple, low cost local implementation circuit of reduced complexity.
There remains a further need for a system which can provide a radio link using conventional wireless technology including transceivers capable of full duplex operation, and a base station which does not require special circuitry for transmitting the dialed telephone number to the PSTN. Accordingly, it is an object of the present invention to provide such an improved arrangement which is of lower cost and reduced complexity.